Figure 1.

Mechanisms for organ size control. (a) Organ formation, exemplified here by leaf development, consists of two stages. The first phase is underpinned by cell proliferation, characterized by intense macromolecular/cytoplasmic synthesis and rapid cell division. The second phase is characterized by cell expansion and differentiation. Differentiation takes place along a basipetal gradient (that is, from leaf tip to leaf base), as indicated here by the gradient in cell size and cell greening. The red arrow summarizes the proliferative inputs, and the black arrow the arrest of proliferation and initiation of differentiation. (b, c) The two principal mechanisms for controlling organ size. Enlargement of organs can be produced by either (b) increasing proliferation signals or (c) delaying the transition between proliferation and differentiation. In both cases the number of cells available for organ formation at the end of the proliferative phase is increased, but the underlying mechanisms are different.